Methods and systems for detecting physiology for monitoring cardiac health

ABSTRACT

In one aspect, a photoplethysmograph system to measure a user&#39;s heart rate includes one or more light-emitting diodes (LED) that provide a constantly-on light signal during a measurement period. The one or more light-emitting diodes are in optical contact with an epidermal surface of the user. The one or more light-emitting diodes emit a light signal into the tissue of the user, and wherein the tissue contains a pulsating blood flow. A light-intensity sensor circuit converts the reflected LED light from the tissue into a second signal that is proportional to a reflected light intensity. The second signal includes a voltage or current signal. A computer-processing module calculates the user&#39;s beat-to-beat heart rate from the second current signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application hereby incorporates by reference the followingapplications in their entirety: U.S. Provisional Patent Application No.62/086,910, titled METHODS AND SYSTEMS FOR DETECTING PHYSIOLOGY FORMONITORING CARDIAC HEALTH and filed on 3 Dec. 2014.

1. FIELD

This application relates generally medical devices, and moreparticularly to a system, apparatus and method of measuring a number ofphysiologic parameters which can be used for quantification of heartfailure status.

2. BACKGROUND

Overall, 5.8 million individuals in the United States suffer from heartfailure (HF) and one in ten elderly develops HF. Despite advances intreatment and relative improvement in survival, the rate of HFhospitalizations has surpassed one million yearly with HF becoming theleading hospital diagnosis for Medicare patients. The costs for HF careare close to $40 billion annually and this represents a large cost tothe Medicare system. More than two-thirds of the costs are related to HFhospitalizations, as a result of suboptimal disease management, withroughly 25% of patients readmitted within 30-days of hospital discharge.

A major barrier in preventing HF related hospitalization is the currentreactive standard of care, which involves relying on a patientself-reported daily weight to determine if a sudden increase in bodyfluid weight has occurred. Unfortunately, body weight has been shown tobe an unreliable marker for cardiac decompensation, and suffers fromvery low patient compliance. It is worth emphasizing that the populationof individuals who suffer from heart failure are predominantly elderlyand thus have a very hard time following instructions, often forgettingtheir tasks or are unmotivated to follow instructions.

At least fifty percent (50%) of HF related hospitalizations arepreventable, and that early detection of symptoms can led to a fifty-sixpercent (56%) reduction in mortality in this population, simple andreliable non-invasive methods to detect early HF decompensation aremissing. New tools for HF outpatient monitoring and management canreduce its high morbidity.

Several studies conducted in HF patients that have cardiac implantableelectronic devices (CIED) capable of monitoring physiologic parametershave shown that trending these parameters can predict a HFdecompensation before it occurs. Unfortunately, such devices are used inless than a third of HF patients, require a surgical procedure toimplant the device and need in home remote setup, adding to thecomplexity of the disease management process.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a bioimpedance spectrometer system includes twocurrent-delivery electrodes that convey an alternating current (AC)signal through a user's tissue, wherein the two current-deliveryelectrodes are placed in contact with a surface of the user's tissue.The bioimpedance spectrometer system includes two sense electrodes thatmeasure the differential voltage on the tissue. An instrumentationamplifier measures the differential voltage on the surface of the user'stissue through the two sense electrodes, and generates a voltagemeasurement signal corresponding to the difference in voltage betweeneach of the two sense electrodes. A low-impedance or high-impedancecurrent source circuit maintains a specified-range value of thealternating current (AC) current passing through the user's tissue suchthat the magnitude of a tissue impedance is equal to a differentialvoltage between two sense electrodes on the tissue divided by a known ormeasured current provided by the current source circuit. A processingmodule calculates an impedance magnitude or a complex impedance valuecalculated from the impedance magnitude and a phase shift between thedifferential voltage and the AC current and determines a relative amountof Intracellular and extracellular fluid from the impedance magnitude orcomplex impedance value.

In another aspect, a photoplethysmograph system to measure a user'sheart rate includes one or more light-emitting diodes (LED) that providea constantly-on light signal during a measurement period. The one ormore light-emitting diodes are in optical contact with an epidermalsurface of the user. The one or more light-emitting diodes emit a lightsignal into the tissue of the user, and wherein the tissue contains apulsating blood flow. A light-intensity sensor circuit converts thereflected LED light from the tissue into a second signal that isproportional to a reflected light intensity. The second signal includesa voltage signal or a current signal. A computer-processing modulecalculates the user's beat-to-beat heart rate from the second currentsignal.

In yet another aspect, a computerized system for measuring one or morephysiologic parameters used to quantify a cardiac state related to heartfailure includes a heart rate sensor means that measures the physiologicparameters used to quantify the cardiac state related to a heart failurestate. A telemetry system means that communicates the physiologicparameters to a data analysis means. A data analysis means that receivesthe physiologic parameters and determines a cardiac state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example system for implementing variousembodiments.

FIG. 2 depicts an example of a photoplethysmograph system used tomeasure heart rate, according to some embodiments.

FIG. 3 illustrates an example photoplethysmograph system used to measureheart rate, according to some embodiments.

FIG. 4 A-B illustrates and example use of bioimpedance spectrometersystem, according to some embodiments.

FIGS. 5 A-C illustrates example implementations of aspects of abioimpedance spectrometer, according to some embodiments.

FIG. 6 illustrates a block diagram of example circuits for measuringbioimpedance, according to one example embodiment.

FIG. 7 depicts a strap that can incorporate the PPG and bioimpedancecomponents to have optical contact to the skin, according to someexamples.

FIG. 8 is an example of an alternative method of measuring the compleximpedance of a signal, according to some embodiments.

FIG. 9 illustrates an example of a system that can be used to implementa photoplethysmograph (PPG) system, according to some embodiments.

FIG. 10 depicts an exemplary computing system that can be configured toperform any one of the processes provided herein.

The Figures described above are a representative set, and are not anexhaustive with respect to embodying the invention.

DESCRIPTION

Disclosed are a system, method, and article of manufacture for measuringa number of physiologic parameters that can be used for quantificationof heart failure status. The following description is presented toenable a person of ordinary skill in the art to make and use the variousembodiments. Descriptions of specific devices, techniques, andapplications are provided only as examples. Various modifications to theexamples described herein can be readily apparent to those of ordinaryskill in the art, and the general principles defined herein may beapplied to other examples and applications without departing from thespirit and scope of the various embodiments.

Reference throughout this specification to “one embodiment,” “anembodiment,” ‘one example,’ or similar language means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the presentinvention. Thus, appearances of the phrases “in one embodiment,” “in anembodiment,” and similar language throughout this specification may, butdo not necessarily, all refer to the same embodiment.

Furthermore, the described features, structures, or characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. In the following description, numerous specific details areprovided, such as examples of programming, software modules, userselections, network transactions, database queries, database structures,hardware modules, hardware circuits, hardware chips, etc., to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art can recognize, however, that the invention may bepracticed without one or more of the specific details, or with othermethods, components, materials, and so forth. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

The schematic flow chart diagrams included herein are generally setforth as logical flow chart diagrams. As such, the depicted order andlabeled steps are indicative of one embodiment of the presented method.Other steps and methods may be conceived that are equivalent infunction, logic, or effect to one or more steps, or portions thereof, ofthe illustrated method. Additionally, the format and symbols employedare provided to explain the logical steps of the method and areunderstood not to limit the scope of the method. Although various arrowtypes and line types may be employed in the flow chart diagrams, andthey are understood not to limit the scope of the corresponding method.Indeed, some arrows or other connectors may be used to indicate only thelogical flow of the method. For instance, an arrow may indicate awaiting or monitoring period of unspecified duration between enumeratedsteps of the depicted method. Additionally, the order in which aparticular method occurs may or may not strictly adhere to the order ofthe corresponding steps shown.

DEFINITIONS

Bioimpedance can be a measure of the impedance of tissue.

Photoplethysmograph can be a device used to optically obtain avolumetric measurement of an organ. For example, the volume of a bloodvessel.

Exemplary Systems, Use Cases and Computer Architecture

FIG. 1 illustrates an example system 100 for implementing variousembodiments. System 100 can be a user-wearable computing system. System100 can include various sensor systems that obtain various physiologicmetrics. Said physiologic metrics can be used to predict cardiacdecompensation (e.g. worsening heart failure state). Example physiologicmetrics include, inter alia: heart rate (HR), heart rate variability(HRV); activity levels; respiration rate (RR); pitting edema (e.g.peripheral edema); etc. Some metrics can be measured directly, such asheart rate, which are then used to derive other metrics, such as heartrate variability.

In one example, system 100 can be a non-invasive system for ambulatorymonitoring of heart failure (HF) patients. System 100 can be utilized topredict a HF exacerbation before it manifests in a hospitalization event(e.g. several days, several hours, etc.). System 100 can include systemsto alert patients, caregivers and healthcare providers, and/or enableproactive treatment while patients remain at home or anotherenvironment. System 100 can include various hardware sensors (and theirconcomitant driver systems) to, inter alia, monitor relevant physiologicmetrics (e.g. see infra) such as peripheral edema. In one example,system 100 can be worn on different parts of the body (e.g. wrist, otherareas of the arms, ankles, head mounted, chest, etc.). System 100 thathas either capacitive or galvanic contact with the epidermis of the user(e.g. ECG electrodes) as well as optical contact (e.g. fingerblood-oxygen saturation monitor). Example locations can be convenientand comfortable for the user when worn but also have access to measurevascularization and/or various areas where edema can occur. System 100can also incorporate various wireless technology standards forexchanging data of over short distances (e.g. short-wavelength Ultrahigh frequency (UHF), Bluetooth®, ANT, ANT+, Wi-Fi, and/orultrasonic-based telemetry systems, etc.). System 100 can alsoincorporate a wireless module enabling communications over a cellularnetwork (e.g. GSM module)

System 100 can include a user interface to elicit user information andsymptoms. System 100 can include a user interface that providesinformation and/or feedback to the user. System 100 can include a userinterface that provides bidirectional communications. This can beincorporated onto the device itself as a simple user interface (i.e.LED/OLED display with buttons), though a web portal, through asmartphone application, augmented-reality elements in a head-mounteddisplay and/or through a stand-alone device. System 100 can include theability to ask specific questions to a user and thus, increase thesensitivity of the system and decrease the rate of false positives.Rather than, or in addition to, a user interface, especially forindividuals who suffer from dementia or are otherwise incapable orinteracting with hardware, a call center can call the Individuals whenit is detected that the hardware is not being worn properly or when themeasured data demonstrates some level of cardiac decompensation andfurther data can be collected to ensure a low false-positive (or for anyother reason), or to provide instructions to the individual.

System 100 can include one or more temperature sensors 111 to measureskin and/or ambient temperature. The skin temperature sensors can beexposed through the device to make contact to the skin. The skintemperature sensors can be coupled via a highly temperature conductivematerial to the skin and/or in close proximity to the skin. The skintemperature sensors can be encapsulated in the same material as rest ofsystem 100. Additionally, the ambient-temperature sensor can be exposedthrough the system 100 to make contact to the air. Theambient-temperature sensor can be coupled via a highlytemperature-conductive material to outside surface of the device and/orin close proximity to the outside of the device encapsulated in the samematerial as rest of system 100. The sensors can consist of a dedicatedintegrated circuit (e.g. MCP9700), a thermistor, a thermo-couple withamplifier, a passive infrared sensor, a thermopile, etc.

System 100 can include one or more processors to implement algorithms tocombine the measured data into a clinically-relevant predictor ofworsening HF (e.g. a cardiac decompensation event). When adecompensation event is detected, system 100 can notify the user byproviding feedback through the device itself (e.g. display and/or LEDs,audio alerts, haptic alerts, etc.), though a smartphone (e.g. viatelemetry), through an appliance, through a web portal and/or by a callcenter contacting the patient and/or the patients care-giver orphysician contacting a specified care giver.

System 105 can Include the user-wearable aspect of system 100. System105 can include various miniature electronic devices that are worn bythe bearer under, with or on top of clothing (e.g. such as thoseprovided in FIG. 1). Various information obtained by the subsystems ofsystem 105 can be communicated to wireless communication and/or uplinkdevice 106 (e.g. for storage in a data store, further processing, etc.).System 108 can be another computing system such a personal computer,laptop computer, tablet computer, smart phone, head-mounted displaycomputer, etc. System 108 can also include any Wi-Fi access point.System 108 can also include a device display 107 for providinginformation to a user. Remote services 109 can include additionalcomputerized-services such patient monitoring services, medical-providerservers, insurance servers, medical-study servers, etc. Remote services109 can use the measured physiologic data to compute the probability ofcardiac decompensation. Remote services 109 can communicate with system105 and/or system 108 via the Internet 110. Various examples of remoteservices are provided herein. In some embodiments, various portions ofsystem 105 and system 108 can be integrated into a single device and/orbe implemented as a virtual computing system(s). In some embodiments,various portions of system 105, system 108 and/or remote services 109can be implemented in a cloud-computing environment. In one example, 105and/or 108 can relay data to the cloud via a direct Wi-Fi connection, awireless data link to a smartphone, an appliance or personal computerapplication. The smartphone, appliance or personal computer applicationcan provide feedback. The feedback can be generated from various serversimplemented in the cloud (e.g. a cloud-computing platform) for displayto the end user, or can prompt the user for additional informationand/or other type of interface.

Photoplethysmograph 113 can optically obtain a volumetric measurement ofan organ, in particular, the blood vasculature in tissue. As the heartbeats, pulses of blood are sent through the arteries and into thecapillaries. These expand under the pulsatile pressure, reducing thetransmission of light propagating through the tissue.Photoplethysmograph 113 can sense the variations in light transmissioncorresponding to the variation in wave caused by these pulsatilepressure changes. The rate of the arrival of these waves can beinterpreted by system 100 as the heart rate. For example, system 100 canmeasure the number of waves over a period of time (e.g. number of wavesover 10 seconds*6=heart rate in beats per minute) and/or by measuringthe time between any two fiduciary points (e.g. minima, maxima, zerocrossing, maximum slope, minimum slope, or derived from a wavelettransform etc.) and computing the inverse to derive pulse rate.Accordingly, heart rate variability can be determined by variousmethods. For example, it can be computed as the standard deviation ofthe interpulse or interbeat interval (time between two successivepulses) over a period of time. The accuracy of the derived measure canbe a function of the accuracy by which the interbeat can be measured.Other examples include the standard deviation of 5 minute mean heartrate values over 24 hours, or a frequency analysis of several minutes'worth of interbeat intervals and measuring the ratio of energy in aparticular frequency band (e.g. 0.15 to 0.4 Hz) to that in another band(e.g. 0.04 to 0.15 Hz). System 100 can determine an interbeat interval.An example photoplethysmograph design that can be utilized forphotoplethysmograph 113 is provided infra.

System 100 can utilize this information to also determine a respirationrate. For example, two methods for determining respiration rate are nowprovided. System 100 can utilize either one, or a combination of thetwo. One method relies on the respiratory sinus arrhythmia (RSA) that isthe parasympathetically mediated slowing down of the heart rate duringexhalation and speeding up during inhalation. A plot of theinstantaneous heart rate (e.g. a tachogram) can be correlated to arespiratory rate. The respiration rate is then measured the same way theheart rate is measured from this waveform (e.g. peak to peak interval).

A secondary method can include examining the low frequency component ofthe photoplethysmograph. Variations in pulmonary pressure (e.g. as aresult of respiration) can result in minute blood pressure variationsand can be picked up by the photoplethysmograph. By filtering thephotoplethysmograph for the frequencies that correspond to respiration,a waveform corresponding to respiration can be detected and used tomeasure respiratory rate.

Bioimpedance spectrometer 101 can be used to measure the compleximpedance and/or opposition to the flow of an electric current throughthe user's tissue. For example, bioimpedance spectrometry can be used toquantify fluid compartmentalization. For example, bioimpedancespectrometer 101 can quantify/measure peripheral edema (e.g. pittingedema). Worsening peripheral edema can be utilized as a predictor ofdecompensation in HF patients and is thus a method of characterizing thestate of cardiac decompensation. For example, bioimpedance spectrometer101 can be incorporated into system 100 implemented as a user-wearabledevice. Example implementations of various bioimpedance spectrometersare provided infra. It is noted that in some examples, in lieu ofcomplex impedance the magnitude of the impedance can be utilized.

Bioimpedance spectrometer 101 can be used to measure heart rate. 101 canbe operated at a fixed frequency and the change in the magnitude of theimpedance can reflect the volumetric changes associated with bloodvessels/capillaries expanding as the pulsatile pressure wave arrivesfrom the heart as a result of the heart beating (bioimpedanceplethysmography). Thus, by measuring the impedance changes in thetissue, it is possible to measure hear rate, akin to the method used bythe photoplethysmogram 113. It is also possible to combine the output ofthe photoplethysmogram and the bioimpedance plethysmograph (or otherheart rate sensing technologies, etc.) to derive a better estimate ofthe actual heart rate through a number of methodologies, such as theapplication of a Kalman filter to the signals.

Device display 102 and/or 107 can be an electronic visual display orLEDs, output device for presentation of information for visual ortactile reception (e.g. a touchscreen, etc.).

Accelerometer 112 can be a device that measures proper accelerationand/or rate of change of velocity. Accelerometer 112 can detect theorientation of system 100. For example, user activity, as well as typeof activity, can be quantified with a three-axis accelerometer that isincorporated into the hardware. It is noted that in some exampleembodiments, a piezo-electric sensor can be used to detect heart rate(e.g. pulse rate).

For example, accelerometer 112 can be a one (1), two (2), or (3) axisaccelerometer. Accelerometer 112 can be an integrated accelerometeralong with other sensors (e.g. gyroscopes, compasses, etc.) and/ormicrocontrollers. Accelerometer 112 can interface a microcontroller 103(or other type of processing unit) using an analog and/or a digitalinterface (e.g. I²C (Inter-integrated Circuit), Serial PeripheralInterface Bus, etc.). Accelerometer 112 can be configured to detectcertain types of motion without having to measure the outputcontinuously, and the microprocessor is notified when an interrupt istriggered (by changing the output voltage on a shared line). This allowsmicrocontroller 103 to enter a low power mode and detect when motionoccurs.

Accelerometer 112 can provide motion detection for activityquantification. Accelerometer data can be converted to activityinformation. An example low-power implementation can utilize a built-inmotion detection mechanism to trigger an interrupt that is detected bythe microcontroller and used to increment an internal counter. System100 can record a number of interrupts over configurable duration oftime, such as fifteen (15) minutes. The activity level of the user vs.time can then be equal to the number of interrupts recorded in these‘buckets’. Information obtained by any system/device of system 100 canbe stored in a local memory and/or remote data storage system (notshown).

Accelerometer 112 can provide motion detection for artifact mitigation.In order to ensure that minimal motion artifacts interrupt thephotoplethysmograph and/or bioimpedance measurements, system 100 canfirst detect a period when there is minimal motion. This is accomplishedby starting a timer on microcontroller 103, and waiting for either aperiod of time to lapse or motion to be detected by the interrupt. Whenmotion is detected before the requite time, then may be too muchactivity to perform a measurement. System 100 can then wait for a periodof time and attempt to detect a period of low activity again. When anactivity is not detected, the measurements can commence. If motion isdetected during a measurement, then the measurement can be aborted andthe system can wait until another period of inactivity. System 100 canpredict times that are more opportune for a measurement by keeping trackof periods of activity and inactivity over time and computingprobabilities of user activity as a function of time or day and/or dayof the week. By selecting times to perform measurements that have ahigher probability of inactivity, system 100 can conserve power by onlytesting for inactivity during these periods.

Accelerometer 112 can be used to detect the vibrations resulting fromthe pulse, especially at a prominent location like the wrist or neck.Small vibrations that occur at a frequency similar to that of the pulsecan be used as measure of pulse rate. This measure can be combined withthat from the photoplethysmograph or the impedance plethysmograph toderive a better approximation of pulse rate through various techniquessuch as the application of a Kalman filter.

Wireless communications can be managed by wireless data transfer module104. Wireless data transfer module 104 can include any datacommunication systems (e.g. Wi-Fi systems provided supra, cellular dataservice, mobile satellite communications, wireless sensor networks,etc.).

In other example embodiments, system 100 can include an ECG sensor(s)(not shown for reasons of brevity). ECG sensor(s) can measure electricalactivity of the heart. Utilizing the bioimpedance electrodes and/oradditional electrodes on the surface of the device (e.g. on the strap ofthe device, etc.) a user can make contact with the electrodes in variouspositions (e.g. touching the electrodes with two different hands, etc.).

FIG. 2 depicts an example of a photoplethysmograph system 200 used tomeasure heart rate, according to some embodiments. FIG. 2 demonstratesthe path that light can take as it travels from the LED to a photodiode,passing through light absorbing tissue which modulate the lightintensity as a function of tissue volume (e.g. the varying bloodvolume). Photoplethysmograph system 200 can include one or more lightemitting diodes (LEDs) 201 and/or 203 and one or more PIN photodiode(s)202 (or other light sensing device, such as a PN photodiode or aphototransistor). PIN photodiode 202 can include a semiconductor devicethat converts reflected light 205 from emitted LED light 204 into acurrent or voltage proportional to the light intensity. The currentsignal can then be measured and interpreted (e.g. as the user's heartrate). As noted supra, as the heartbeats, pulses of blood are sentthrough the arteries and into the capillaries. These expand under thepulsatile pressure, decreasing the transmission and/or reflection oflight propagating through the tissue. Photoplethysmograph system 200 cansense the variations in light transmission corresponding to thevariation in the pulse wave caused by these pulsatile volume changes. Inone example, the variations in reflected/transmitted light intensity canbe measured. Photoplethysmograph system 200 can be placed in contact(e.g. optical contact) with a user's tissue 206 (e.g. on a write, ankle,etc.). In one example, only a single frequency of LED light can be used,originating from one or more LEDs. The LED(s) can be set to provide aconstantly-on light source during a measurement period, unlike otherimplementations that pulse the light on and off. This allows for thesignal processing electronics to have a very narrow bandwidthcorresponding to the frequency band that is physiologically relevant(e.g. the range of frequencies that correspond to the pulsatile wave)such that the external noise (e.g. external light, motion) can befiltered out, yielding a greater signal to noise ratio.

FIG. 3 illustrates an example photoplethysmograph system 300 used tomeasure heart rate, according to some embodiments. FIG. 3 demonstratesthat any distribution of LEDs in space relative to the photodiode ispossible, as long as the light intensity is sufficient to scatter in thetissue and reach the photodiode. Photoplethysmograph system 300 caninclude arrangement 304 of one or more light emitting diodes (LEDs) 301,302, 303 and a PIN photodiode 300. As noted in FIG. 3, as the distancebetween the PIN photodiode 300 and the LED (e.g. distance 305 betweenLED 302 and PIN photodiode 300) increases, the light travels through agreater amount of tissue and thus is modulated a greater extent by thepulsating blood. However, the increased distance reduces the intensityof light received by the photodiode. In some embodiments, multiple LEDs301 302 and 303 can be used simultaneously around the photodiode toincrease the amount of tissue that Is sampled, thus, increasingsignal-to-noise ratio (SNR). In some embodiments, higher light intensityfrom one or more LEDs placed further from the PIN photodiode willincrease the SNR.

FIG. 4 A illustrates an example of the mechanism by which bioimpedancespectrometry works. When a current source 400 injects a current throughtissue 406 via electrodes 401 and 402, the path the current will takedepends on the frequency of current. At low frequencies, the currentpredominantly travels through the extracellular fluid 404 and is blockedby the lipid bilayer in cell membranes 405, whereas at high frequencies,it travels through the lipid bilayer and through the cells 403, reducingthe overall impedance. Accordingly, these cell walls can be modeled ascapacitors that block low frequency current, but allow high frequencycurrent to pass through. As the frequency increases, more current canflow across the cell walls and through the intracellular fluid. Thisincrease in effective fluid has an effect of decreasing the impedance ofthe current. By measuring the impedance at different frequencies, theratio of fluid outside to inside the cells can be quantified, and can bea measure of edema. For example, the level of edema can be quantified asthe ratio of fluid outside the cells to the fluid inside the cells.

FIG. 4 B illustrates an example of bioimpedance spectrometer system,according to some embodiments, which operates with two electrodes. Avoltage source 410 generates a voltage (Vo) that passes throughelectrode 416 into tissue 415, through electrode 414 across resistor 413and back to the voltage source. The voltage drop across resistor 413 (R)is measured (e.g. by instrumentation amplifier 411) to produce voltage412 (V₂). The impedance of the combination of electrodes 416 and 414,and tissue 415 can be determined the relationship (V₂(Vo−V₂))/R. This isuseful in a number of situations (e.g. if the impedance of theelectrodes 416 and 414 is much less than the tissue impedance, or if theimpedance of the electrodes is constant and the impedance of the tissueis expected to change) but has the disadvantage of allowing theimpedance from the electrode skin interface to affect the measurement.It is noted that in some embodiments, various other amplifier systems(e.g. differential amplifiers, a combination of operational amplifiers,etc.) can be used in lieu instrumentation amplifiers.

FIG. 4C illustrates an example use of bioimpedance spectrometer system101, according to some embodiments, which operates with 4 electrodes anda low impedance current source. Bioimpedance spectrometer system can beplaced in contact with a user's tissue as shown through electrodes 421,425, and a pair of sense electrodes 422 (this is referred to as atetrapolar configuration). Bioimpedance spectrometer system can generatean alternating current (AC) 420 and convey the current throughelectrodes 421 and 425 through the tissue. An instrumentation amplifier411 measures the induced voltage on the skin through pickup electrodes422 and generates a voltage 424 corresponding to the difference involtage between the electrodes 422. The amplifier must have a high inputimpedance to limit the current flow across electrodes 422 that preventsthe impedance of electrodes 422 from effecting the measurement. In thisconfiguration, the effects of electrodes 402 and 403 also do notcontribute to the measured impedance. The high impedance current source420 maintains a constant current passing through the tissue and themagnitude of the tissue impedance is equal to the differential voltage424 divided by the fixed, known, current. The complex impedance can becomputed from the impedance magnitude and the phase shift between themeasured voltage 406 and the current waveforms 401.

The frequency of the fixed alternating current (AC) can be modified(e.g. from a low frequency (e.g. 1 KHz) to a high frequency (e.g. 1MHz)) and the measured impedance can be sampled for at variousfrequencies. By varying the frequency of the AC current 420, therelationship between impedance and frequency can be measured. FIG. 4Dillustrates an example use of bioimpedance spectrometer system 101,according to some embodiments, which operates with 4 electrodes and ahigh impedance current source. The operation of this version of thedevice works the same as in FIG. 4C, however, the tissue and electrodeimpedance can have an effect on amount of current flowing. Tocompensate, a measure of the current must be made. FIG. 41 is an exampleof the addition of a current sense resistor 431 of known fixed value(Rsense), and a mechanism to measure the voltage across the resistor 430(e.g. instrumentation amplifier) resulting in a voltage 432 (Vsense) tothe example in FIG. 4C. In this case, the current is determined by therelationship (Vsense/Rsense). The tissue impedance can thus be computedas the differential voltage across 422 divided by the (Vsense/Rsense).

Bioimpedance spectrometer system 101 can include one or more currentsources, voltage sources, and one or more voltage sensors.

By analysis of the measured impedance, (e.g. a fitting impedance to acole-cole plot and extrapolating the impedance at DC and at infinitefrequency, or by comparing the magnitude of the impedance at low andhigh frequencies), the relative amount of intracellular andextracellular fluid can then be computed.

FIGS. 5 A-B illustrates example implementations of aspects of abioimpedance spectrometer, according to some embodiments. Bioimpedancespectrometry can be the measure of the impedance (e.g. complex impedanceand/or magnitude of impedance) of tissue over a single or range offrequencies. FIG. 5A is an example using a relative gain/phasemeasurement IC 502 (e.g. Analog Device's AD8302) in measuring theimpedance. For a current 501 that flows through two unknown impedances503 and 504 (e.g. resistors), by measuring the differential voltageacross the impedance elements and optionally amplifying the signalthrough instrumentation amplifiers 505 and 506. The signals can beconditioned (e.g. filtered, scaled, etc.) and applied to the gain/phasemeasurement IC 502 for measurement of the relative amplitude and phasedifference between the signals. The ratio of amplitudes of the inputsignals can be converted to a voltage that can be measured 507 andquantified by a microcontroller or analog to digital converter. Thephase difference between the signals can be converted to a voltage 508which can be also be measured by a microcontroller or analog to digitalconverter. If one impedance element is known, value of the otherimpedance element can be computed. One impedance element 503 or 504 canbe replaced by tissue to enable the measurement of the impedance of thetissue. FIG. 5B demonstrates the replacement of 504 by tissue 510. Thetissue in either case is connected to four electrodes, as depicted bythe arrows in the figure. The system measures the bioimpedance of thetissue between the inner pair of electrodes that are connected to theamplifier, (e.g. 511). The outer two electrodes represent the currentsource and sink electrodes that may be connected directly to the skin,or AC coupled via one or more capacitors.

More specifically, FIG. 5A depicts a block diagram describing themethodology by which the gain/phase measurement IC 502 can measure anunknown impedance. ‘I’ 501 represents the current passing through thetwo impedances. Z1 503 & Z2 504 represent the known and unknownimpedances, respectively, A1 505 and A2 506 can represent twoinstrumentation amplifiers with gains G1 and G2 which output a voltageV1 and V2 that is passed to the relative gain/phase measurement IC (e.g.AD8302). The IC can measure the relative gain and phase, and output it(e.g. the AD8302 has output X1 507, a voltage proportional to the logratio of input amplitudes (e.g. a log based ten ratio of the V1 over V2)and X2 508, a voltage proportional to the phase difference in the inputsignals). The unknown impedance Z2 504 can thus computed as(G1*Z1)/(G2*10̂(X1)).

As stated, in some embodiments, the current source does not need to below impedance in nature, thus simplifying the design. One embodiment ofa current source that does not require low impedance output can be madeby a waveform generator (e.g. direct digital synthesis (DDS) IC) that iscapable of generating a sine wave with a configurable frequency and oramplitude. The DDS output can be amplified, filtered, and/or scaled(e.g. passive or active filtration, through dedicated ICs or one or moreoperational amplifiers). The signal can then be fed through a currentlimiting resistor (Z known), effectively setting the impedance of thecurrent source to the value of the current limiting resistor. Thecurrent limiting resistor can be the known impedance resistor 503, or itcan be a separate resistor or combination of resistors. Output impedancecan range from a few kilo ohms to mega ohms. The resulting current canbe calculated as I=V/(Zknown+Ztissue+Zelectrodes) where I is thecurrent; V is the output amplitude of the waveform generator; Zknown isthe impedance of the current limiting resistor; Ztissue is the tissueimpedance which varies with frequency; and Zelectrodes is the resistancecontribution from the electrodes (402 & 403) used to interface thetissue. Thus, one can set the maximum current by selecting V(amplitude), Zknown, and Zelectrodes. The actual current will always belower due to the addition of Ztissue, so this represents the maximumvalue. As the tissue impedance varies, the current can vary.

The waveform generator can take on various forms, from a number ofpassive components forming a tunable oscillator to a voltage-controlledoscillator. A sample implementation can use a direct digital synthesis(DDS) integrated circuit (IC). The DDS IC can use a lookup table and anintegrated digital-to-analog converter (DAC) to generate a sine wave.The output of such as signal can be buffered, in some examples, filteredto remove undesired frequency components.

FIG. 6 illustrates circuits 600 for measuring bioimpedance according toone example embodiment. A DDS 605 (e.g. the AD9837) can be controlled bya microcontroller 604 (or FPGA, CPLD, processor, etc.) to output avarying frequency. The signal can then be buffered and filtered 606 toremove any undesired frequency components (e.g. DDS switching noiseabove 3 MHz). By removing the high frequency components, the waveformcan be smoothed out such that it closer resembles a sine wave. Thiswaveform can then be sent through a DC blocking capacitor 607 followedby the known impedance 608 (e.g. a non-inductive resistor with only realimpedance). In one example, the output waveform 613 can be approximatelythree-hundred (˜300) mV in amplitude. The current can be set by theequation I=V/R; where V is controlled by the DAC and gain of thefiltering/buffering amplifier 606. R can be the series combination ofthe fixed impedance 608, the source and sink electrode impedance, thetissue/electrode interface impedance and the tissue impedance 609.

The sink 614 for this current can be the DC voltage set by the high passfilter 607 that represents the virtual ground (e.g. the mid-railvoltage, or Vcc/2). This can also act as a bias for the tissue ensuringthat the signal is centered within the operating range of the inputamplifiers. Alternatively, DC blocking capacitors can be added to theinputs of the amplifier 610 as well as between the tissue 609 and ground614 to allow the voltage of the tissue to float independent to thevoltages found in the bioimpedance spectrometer system 101.

A set of electrodes can measure the induced voltage in the tissue as aresult of the current and amplifies it with instrumentation amplifier610. Another instrumentation amplifier 611 can measure and optionallyamplifies the current passing through the known resistor. Theseamplifiers can be high-speed instrumentation amplifiers 601 capable ofoperating linearly up to a 1 MHz. These voltages can be scaled up ordown by altering the gain of the amplifiers and/or by adjusting thevoltage dividers that follow the amplifiers.

In one example, three (3) high-speed operation amplifiers 602-603 can beused in the place of the instrumentation amplifier 611 or 610. Twoamplifiers 602 can be used as buffers to maintain high input impedanceand the third amplifier 603 is used in a differential configuration.These amplifiers can be configured to provide any required gain and/oras unity gain. This configuration can enable a low power/low voltageoperation while operating linearly (e.g. gain-wise) up to one (1) MHz.Additional filtering can be implemented before, after, or in conjunctionwith the amplifiers. This can include radio-frequency filtering and/orother type of filtering that counteracts environmental noise. Forexample, a high pass filter in the order of three-hundred (300) Hz canbe used to block 50 Hz or 60 Hz line noise from being introduced intothe circuit. Additionally, a radio-frequency interference (RFI) filtercan be used to block RF interference from the wireless system. Thesystem must be designed such that the input voltages to the relativeamplitude/phase detector IC are within the operating range of the IC. Inthe case of the AD8302, this range is from 223 uV to 223 mV. The gainsprovided by amplifiers 611 and 610 can be set to ensure that the signalfalls within this operating range.

Other configurable parameters (for the AD8302, for example) includeadjusting the mapping (e.g. slope) of output voltages 615 of thegain/phase detector 612 for a given input amplitude/phase difference.These parameters can be tuned to maximize sensitivity over the widestphysiologically possible range present in the population. The output ofthe gain/phase detector 612 can then fed to the microcontroller 604 thatdigitizes the output using an analog digital converter (ADC). Analog ordigital filtering can be applied to the output of the gain/phasedetector 612. For example, several measurements can be made and averagedto reduce the output noise in the system. Also, the step size betweendifferent frequencies can be adjusted to produce more (and/or less) datapoints. Once the data has been collected, it can be sent upstream (e.g.via the telemetry system of FIG. 1 supra). Additionally, in someexamples, computations can occur on the unit itself. The relativeamplitude and phase data can be converted to real and Imaginaryimpedance values. In the case of the AD8302 IC as the gain/phasemeasurement IC, this is accomplished by first converting the relativephase and gain output voltages 615 into a phase angle (α) and log ratioof amplitudes, as per the methodology described in the AD8302 datasheet(e.g. through linear or logarithmic mapping). The log ratio ofamplitudes is then converted to a real impedance (Z_(r)). The geometricequation Z_(i)=tan⁻¹ (α)/Z_(r) can then be used to compute the imaginaryimpedance (Z_(i)) from the phase and real impedance.

Once the Real (Zr) and imaginary (Zi) components are known, they can beplotted on a cole-cole plot where the x-axis represents the realcomponent and the y-axis represents the imaginary component of thecomplex Impedance, over the range of frequencies. The curve can beextrapolated to intersect the x-axis at the points Zinf and Zo(extrapolated impedance at infinite frequency, and at DC). Zo can beproportional to the impedance of the extracellular fluid, and Zinf canbe proportional to the impedance of all the fluid. In one example, thefluid status can be tracked as the ratio of the extracellular fluid tothe total fluid. By tracking these variables (or a combination thereof),relative changes in edema can be computed. For example, with multipledays' worth of data acquired while the user is known to have normalfluid levels, a normal level for a user can be determined. Accordingly,any sudden changes to the level of extracellular fluid can be detected(e.g. by a measured increase in the ratio of extracellular fluid tototal fluid).

FIG. 7 depicts a strap 700 that can incorporate the PPG components (e.g.LED 703 and/or photodiode 704 and/or optics) to have optical contact tothe skin, according to some examples. The strap can be configured toisolate the LED(s) and the photodiode from each other (e.g. preventinglight from the led to hit the photodiode without first traveling throughthe subject's tissue). The interface can be a light pipe and/orincorporate optics to help focus the light or increase the surface areaof the photodiode. The LED and/or photodiode can be incorporateddirectly into the strap with either a small Printed circuit board (PCB,Flex PCB) including a portion of theamplification/filtering/conditioning circuit and/or tethered to the maincircuit board with a ribbon cable or other connection mechanism (or thestrap can expose holes for the led/photodiode which can reside on themain circuit board). The strap can also include a number of conductivesurfaces for the bioimpedance circuit 701, 702, 705, 706. In oneexample, at least four (4) points are used which are insulated from eachother. A strap can be composed of an array of conductive dry rubberelectrodes. The strap can be used for the current source/sink and/or twopickup electrodes. Other strap examples can include conductive fabricelectrodes (e.g. can be embedded into a garment as well as a “watchstrap”), exposed metal electrodes, as well as any other type ofelectrode configuration. The measurement can be made while the wrist ismotionless (e.g. as detected by the onboard accelerometer to help reducemotion related artifacts). These conductive surfaces can be rubber,silicon or metal embedded into the strap (or other conductive element).These pads can be spaced out or kept close together, as long as a sweatbridge cannot form between the conductive pads that could short out thepads. The strap can incorporate channels 707 which are raised/depressedrelative to the pads to help prevent sweat bridge formation.

FIG. 8 is an example of an alternative method of measuring the compleximpedance of a signal, according to some embodiments. System 818 is anIQ demodulator. The signal to be analyzed 801 is fed to a switch 802(e.g. multiplexer, etc.) controlled by inputs 815 from amicrocontroller, FPGA, CPLD, counter, etc. The switch will route thesignal to four different outputs 803 804 805 806 depending on the phaseof the signal. When the signal's phase is between 0-90 degrees, theswitch will output the signal to 803. Likewise, at 90-180 degrees,180-270 degrees, 270-360 degrees, etc., it will route the signal to 804805 and 806, respectively. Thus the switch must cycle through theoutputs at a rate of 4 times the input signal 801 frequency and be phaselocked to the input frequency. The different signals 803 804 805 806 arethen averaged on capacitors 807 808 810 809, respectively.Instrumentation amplifiers 811 will produce a voltage 813 proportionalto the difference in voltages from capacitors 808 and 809 thatrepresents the in-phase signal (I). Instrumentation amplifiers 812 willproduce a voltage 814 proportional to the difference in voltages fromcapacitors 807 and 810 that represents the quadrature signal (Q). Thesignals 813 and 814 can then be converted to a numerical value using ananalog to digital converter (not shown). The magnitude of the signal canbe computed as the square root of the sum of the I squared and Qsquared. The phase angle can be computed as the inverse tangent of theratio Q to I.

FIG. 9 illustrates an example of a system that can be used to implementa photoplethysmograph (PPG) system 900, according to some embodiments. Amicrocontroller (or other controlling device) can directly output avoltage through a built-in digital to analog converter (DAC) or anexternal DAC 906. The voltage can be converted to a current (e.g. usinga transistor) 905 which is used to illuminate the LED(s) 910. The lightpasses through and/or is reflected 205 by the tissue to reach thephoto-sensor 901. In one embodiment, the photo-sensor is a PINPhotodiode that is reversed biased to increase sensitivity and decreasecapacitance. In one embodiment, the photodiode can generate a currentproportional to incident light that is converted to a voltage through atransimpedance amplifier 902. The amplifier can be configured tolow-pass filter frequencies that are physiologically relevant (e.g. DC-5Hz) to reduce noise as well as to filter for any high frequency externallight such as fluorescent lighting, or motion. To ensure that the inputcurrent from photosensor 901 (e.g. a photodiode) is within operatingrange of amplifier 902, the amplifier output 908 can be measured by themicrocontroller (or other processing device) 907 and if it is too highor too low (e.g. out of the operating range of 902), the LED 910 currentcan adjusted by varying the output of the DAC 906. If the voltage 908 istoo high (e.g. saturating) then the DAC 906 output can be reduced toreduce the LED current via transistor 905. If it is too low, then it canbe increased via the same mechanism. When the voltage 908 is in thedesired range, is can be high pass filtered 903 so as to keep theaverage signal voltage near the middle of the supply voltage range. Anadditional gain stage 904 can be implemented to further amplify thesignal allowing for better beat detection. The output 909 can be sampledby the microcontroller 907 and either stored, transmitted, or processingapplied to the signal. In some embodiments, filtration can occur usingan active filter design in conjunction with amplifiers and/or variouspassive components (e.g. Independent of the amplifiers).

In some embodiments, PPG 900 photoplethysmograph system can utilize oneLED frequency and the LED can remain illuminated throughout the durationof the measurement. This can allow for optimized analog filtering (e.g.a narrow bandwidth) of the signal of interest. When the LED isconstantly on, then the transimpedance amplifier 902 bandwidth can betuned to the exact bandwidth of the PPG signal (e.g. DC-5 Hz). This cansignificantly reduce the noise entering the system. For example,including just one LED frequency means that a photodiode can be selectedwhich is maximally sensitive at the same frequency of the LED. Thisallows for lower LED current requirements, thus extending the batterylife of the device. An infrared LED can be been chosen further reducingpower consumption as photodiode sensitivities are typically greater forinfrared light. Alternatively a green LED can be used due to its highlevel of absorption by the blood hemoglobin, and a correspondingphotodiode that is maximally sensitive to the same green wavelength. Thedistance between the LED and photosensor/photodiode 901 can be increasedforcing the light to travel through more tissue, increasing the SNR ofthe signal. Typically higher LED current is required to compensate forthe greater LED/photodiode separation. In one example, the distance ofsix-point-three (6.3) mm provide a good compromise between powerconsumption and signal amplitude. Any number of LEDs can be used,according to various embodiments. A photodiode with a very large surfacearea can be used to further increase sensitivity. PPG system 900 isprovided by way of example and not of limitation. Other PPGconfigurations can be utilized in other embodiments.

FIG. 10 depicts an exemplary computing system 1000 that can beconfigured to perform any one of the processes provided herein. In thiscontext, computing system 1000 may include, for example, a processor,memory, storage, and I/O devices (e.g., monitor, keyboard, disk drive,Internet connection, etc.). However, computing system 1000 may includecircuitry or other specialized hardware for carrying out some or allaspects of the processes. In some operational settings, computing system1000 may be configured as a system that includes one or more units, eachof which is configured to carry out some aspects of the processes eitherin software, hardware, or some combination thereof.

FIG. 10 depicts computing system 1000 with a number of components thatmay be used to perform any of the processes described herein. The mainsystem 1002 includes a motherboard 1004 having an I/O section 1006, oneor more central processing units (CPU) 1008, and a memory section 1010,which may have a flash memory card 1012 related to it. The I/O section1006 can be connected to a display 1014, a keyboard and/or other userinput (not shown), a disk storage unit 1016, and a media drive unit1018. The media drive unit 1018 can read/write a computer-readablemedium 1020, which can contain programs 1022 and/or data. Computingsystem 1000 can include a web browser. Moreover, it is noted thatcomputing system 1000 can be configured to include additional systems inorder to fulfill various functionalities. Computing system 1000 cancommunicate with other computing devices based on various computercommunication protocols such a Wi-Fi, Bluetooth® (and/or other standardsfor exchanging data over short distances includes those usingshort-wavelength radio transmissions), USB, Ethernet, cellular, anultrasonic local area communication protocol, etc.

In some embodiments, the low-impedance or high-impedance current sourcecircuit can maintain a specified-range of a 100 uA-100 mA value of theAC current passing through the user's tissue. The bioimpedancespectrometer can propagate the AC current at frequencies from 1 kHz to 1Mhz). The processing module can measure the magnitude of impedance at a1 Khz frequency and a high 1 Mhz frequency. The processing module cancompare a magnitude of the impedance of the tissue at both low (e.g. 1kHz) and high (e.g. 1 Mhz) frequencies and based on these impedancevalues determines the amount of intracellular and extracellular fluidfrom the complex impedance value. The third voltage signal is band-passfiltered to zero point three (0.3 Hz) to five (5) Hertz (Hz) to reducethe noise and external interference of the third voltage signal.

CONCLUSION

Although the present embodiments have been described with reference tospecific example embodiments, various modifications and changes can bemade to these embodiments without departing from the broader spirit andscope of the various embodiments. For example, the various devices,modules, etc. described herein can be enabled and operated usinghardware circuitry, firmware, software or any combination of hardware,firmware, and software (e.g., embodied in a machine-readable medium).

In addition, it can be appreciated that the various operations,processes, and methods disclosed herein can be embodied in amachine-readable medium and/or a machine accessible medium compatiblewith a data processing system (e.g., a computer system), and can beperformed in any order (e.g., including using means for achieving thevarious operations). Accordingly, the specification and drawings are tobe regarded in an illustrative rather than a restrictive sense. In someembodiments, the machine-readable medium can be a non-transitory form ofmachine-readable medium.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A bioimpedance spectrometer system comprising:two current-delivery electrodes that convey an alternating current (AC)signal through a user's tissue, wherein the two current-deliveryelectrodes are placed in contact with a surface of the user's tissue;two sense electrodes that measure the differential voltage on thetissue; an instrumentation amplifier that measures the differentialvoltage on the surface of the user's tissue through the two senseelectrodes, and generates a voltage measurement signal corresponding tothe difference in voltage between each of the two sense electrodes; alow-impedance or high-impedance current source circuit that maintains aspecified-range value of the AC current passing through the user'stissue such that the magnitude of a tissue impedance is equal to adifferential voltage between two sense electrodes on the tissue dividedby a known or measured current provided by the current source circuit;and a processing module that calculates an impedance magnitude or acomplex impedance value calculated from the impedance magnitude and aphase shift between the differential voltage and the AC current anddetermines a relative amount of intracellular and extracellular fluidfrom the impedance magnitude or complex impedance value.
 2. Thebioimpedance spectrometer system of claim 1, wherein the twocurrent-delivery electrodes and the two sense electrodes are in atetrapolar configuration.
 3. The bioimpedance spectrometer system ofclaim 1, wherein the instrumentation amplifier comprises aninstrumentation amplifier, a differential amplifier, or combination ofoperational amplifiers configured to provide a differential voltageoutput.
 4. The bioimpedance spectrometer system of claim 1, wherein afrequency of the AC current is set to specified frequencies.
 5. Thebioimpedance spectrometer system of claim 4, wherein the measuredimpedance magnitude or complex impedance is sampled for at one or morespecified frequencies.
 6. The bioimpedance spectrometer system of claim1 further comprising: wherein the bioimpedance spectrometer systemcomprises a high impedance bioimpedance spectrometer system, and acurrent-sense resistor of a known-fixed value; a differential voltagesensor that measures the voltage across current-sense resistor.
 7. Thebioimpedance spectrometer system of claim 6, wherein the processingmodule fits a measured impedance to a cole-cole plot and extrapolatesthe real impedance at a direct current (DC) and at an infinitefrequency.
 8. The bioimpedance spectrometer system of claim 7, whereinthe processing module measures the magnitude of impedance at a lowfrequency and a high frequency.
 9. The bioimpedance spectrometer systemof claim 8, wherein the processing module compares a magnitude of theimpedance of the tissue at both low and high frequencies and based onthese impedance values determines the amount of intracellular andextracellular fluid from the complex impedance value.
 10. Aphotoplethysmograph system to measure a user's heart rate comprising:one or more light-emitting diodes (LED) that provide a constantly-onlight signal during a measurement period, wherein one or morelight-emitting diodes are in optical contact with an epidermal surfaceof the user, wherein the one or more light-emitting diodes emit a lightsignal into the tissue of the user, and wherein the tissue contains apulsating blood flow; a light-intensity sensor circuit that converts thereflected LED light from the tissue into a second signal that isproportional to a reflected light intensity, and wherein the secondsignal is a voltage signal or a current signal; and acomputer-processing module that calculates the user's beat-to-beat heartrate from the second current signal.
 11. The photoplethysmograph systemof claim 10, wherein in the light-intensity sensor circuit comprises aPIN photodiode, a PN photodiode, a phototransistor or a light-detectingintegrated circuit.
 12. The photoplethysmograph system of claim 11,wherein the PIN photodiode or PN photodiode is reversed biased toincrease sensitivity and decrease capacitance.
 13. Thephotoplethysmograph system of claim 12 further comprising: amicrocontroller that directly outputs a voltage signal through a digitalto analog converter; and a transistor that converts the voltage signalto a current signal that is used to illuminate the one or morelight-emitting diodes.
 14. The photoplethysmograph system of claim 13further comprising: wherein the current signal of the second signal isproportional to an incident light and is converted to a third voltagesignal through a transimpedance amplifier; wherein the voltage signal ofthe second signal is proportional to the incident light and is bufferedand amplified through an amplifier to the third voltage signal; and thethird voltage signal is band-pass filtered to physiologically relevantfrequencies to reduce the noise and external interference of the thirdvoltage signal.
 15. The photoplethysmograph system of claim 14, whereinthe physiologically relevant low-pass filter frequency comprises five(5) Hertz (Hz).
 16. The photoplethysmograph system of claim 15, whereinthe epidermal surface of the user comprises a wrist region.
 17. Thephotoplethysmograph system of claim 16, wherein the one or morelight-emitting diodes are constantly on, then a bandwidth of the filteris tuned to the relevant bandwidth of a PPG pulsatile signal, whereinthe one or more light-emitting diodes comprise one or more infraredlight-emitting diodes, and wherein the light-intensity sensor circuit ismaximally sensitive at a same frequency of the one or morelight-emitting diodes comprise one or more infrared light-emittingdiodes.
 18. A computerized system for measuring one or more physiologicparameters used to quantify a cardiac state related to heart failurecomprising: a heart rate sensor means that measures the physiologicparameters used to quantify the cardiac state related to a heart failurestate; a telemetry system means that communicates the physiologicparameters to a data analysis means; and a data analysis means thatreceives the physiologic parameters and determines a cardiac state. 19.The computerized system of claim 18 further comprising: an edema sensormeans to measure an edema-related physiological parameter used toquantify an edema state, and wherein the data analysis means receivesthe edema-related physiological parameter and determines the edemastate.
 20. The computerized system of claim 19, wherein the edema sensormeans comprises a bioimpedance spectrometer, wherein the heart ratesensor means comprises a either a photoplethysmograph, a bioimpedancespectrometer, an accelerometer, or a piezoelectric vibration sensor, oran Electrocardiography (ECG) system, wherein the computerized systemcomprises a temperature sensor that obtains a user's temperature that isprovided to the data analysis means, and wherein the cardiac status isrelayed to a specified computer application.